What Size ATS Do You Need for a 12.5kVA Generator?

If you want a 12.5kVA generator to start automatically when the Eskom or municipal supply fails, choosing the correct Automatic Transfer Switch (ATS) matters just as much as choosing the generator itself. For background on South Africa’s electricity supply and interruptions, see the Eskom electricity supply information available from Eskom. The 12.5kVA generator ATS size must suit the generator output, supply voltage, phase configuration and the maximum current the connected installation can draw.

For 12.5kVA applications a 40A or 50A ATS may be suitable but there is no one-size-fits-all answer. The right ATS choice depends on the 12.5kVA generators rated current whether the 12.5kVA generator is phase or three-phase and how the installation is arranged.

This guide shows how to find the ATS size for a 12.5kVA generator, in South Africa. It also gives examples for homes, farms, offices and small businesses.

What Does an ATS Do on a Generator?

An Automatic Transfer Switch is the device that transfers the electrical load between the normal utility supply and the generator.

Under normal conditions, the property receives electricity from Eskom or the local municipality. When the utility supply fails, the ATS detects the interruption and signals the generator to start. Once the generator reaches the required voltage and frequency, the ATS transfers the selected electrical load to generator power.

When the utility power comes back and stays steady the ATS moves the load back, to the utility. Lets the generator shut down after the generator cools.

In short the ATS turns a generator from an operated backup machine into an automatic standby power system.

For businesses, farms, security systems and other applications where somebody cannot be present every time the electricity fails, that distinction is significant.

What Size ATS Do You Need for a 12.5kVA Generator?

There is no sensible way to choose an ATS simply by looking at the generator’s kVA rating.

The first step is to determine the generator’s operating voltage and phase arrangement.

A 12.5kVA generator can have very different current characteristics depending on whether it is configured as single-phase or three-phase.

For example, using the standard electrical relationship:

Single-phase current ≈ kVA × 1,000 ÷ volts

At 230V:

12,500 ÷ 230 ≈ 54.3A

That means a 12.5kVA generator in single‑phase mode can produce about 54A at 230V when it is running at its rated apparent power.

For a three-phase generator:

Current ≈ kVA × 1,000 ÷ (√3 × volts)

At 400V:

12,500 ÷ (1.732 × 400) ≈ 18A per phase

This immediately shows why simply saying that a 12.5kVA generator needs a 50A ATS can be misleading.

The correct 12.5kVA generator ATS size depends on the actual generator configuration and the electrical installation.

Typical starting point

Generator configuration	Approximate full-load current	Possible ATS range
12.5kVA, 230V single-phase	54A	63A class may be appropriate
12.5kVA, 400V three-phase	18A/phase	25A–40A class may be appropriate
Smaller selected-load installation	Depends on load	May require a smaller rated device
Installation with higher upstream protection	Depends on design	Must be assessed by electrician

These figures are for understanding the principle, not a substitute for an electrical design.

The ATS must be selected according to the actual generator, protection equipment, cable sizes, load current and installation requirements.

Why a 50A ATS Is Not Automatically Correct

This is one of the most common misunderstandings we see.

A customer may have a 12.5kVA generator and assume that a 50A ATS is automatically the correct choice. That can be wrong.

A 50A device is not automatically capable of carrying the generators rated current just because the generator is labelled as 12.5kVA.

For a 230V single‑phase generator that produces its 12.5kVA the calculated current comes out to about 54A. A 50A transfer device would therefore need careful consideration and would not normally be chosen just because it is “close enough”.

The opposite mistake can also happen. Installing a larger ATS than necessary does not automatically make the system safer.

The transfer switch must coordinate with the generator output, circuit protection and the installation, as a whole.

How to Calculate the 12.5kVA Generator ATS Size

A practical selection process looks like this.

Step 1: Confirm the generator rating

Check the generator’s technical specification sheet.

Do not rely only on the marketing description.

Confirm:

• Rated kVA
• Prime or standby rating
• Voltage
• Number of phases
• Rated current
• Frequency
• Power factor
• Single- or three-phase configuration
• ATS control compatibility

For example, if you are considering a 12.5kVA Hyundai diesel generator, the generator’s technical specification should be checked carefully to confirm its voltage, phase configuration and rated current before selecting the ATS.

Other 12.5kVA models may have completely different configurations.

Step 2: Calculate the generator current

For single-phase equipment:

Amps = VA ÷ volts

For a 12.5kVA, 230V generator:

12,500 ÷ 230 = approximately 54.3A

For a three-phase 400V generator:

12,500 ÷ (1.732 × 400) = approximately 18A per phase

This is why you should never order an ATS from the kVA figure alone.

Step 3: Check the connected load

The generator may not be supplying the entire property.

A home, shop or farm might have a dedicated essential-load distribution board containing:

• Lighting
• Refrigeration
• Security systems
• Wi-Fi and IT equipment
• Selected plugs
• Borehole pumps
• Gate motors
• Air conditioning
• POS equipment

If the ATS is transferring only selected circuits, its rating must be considered against that installation.

Step 4: Check the electrical protection

The ATS, generator breaker, mains protection and cable sizes need to work together.

A transfer switch should not be treated as an isolated component.

The electrician or electrical contractor should verify the protection arrangement, conductor sizes, earthing, neutral arrangement and switching configuration before commissioning the system.

Single-Phase vs Three-Phase ATS for a 12.5kVA Generator

This is another area where buyers can get caught out.

A single-phase generator normally requires a transfer arrangement designed for the relevant single-phase supply.

A three-phase generator requires an ATS capable of transferring the appropriate phases and neutral arrangement for that installation.

A 230V single-phase generator and a 400V three-phase generator require different considerations. If you are unsure which configuration suits your application, our single-phase vs three-phase generator guide explains the practical differences and where each type of generator is normally used.

If a property has a three-phase municipal supply but the generator is single-phase, the installation cannot simply be connected as though both supplies were identical.

The load distribution also matters.

For a three-phase installation, an electrician needs to assess how the loads are distributed across the phases and whether the generator can supply the required circuits without creating an imbalance or overload.

Do not buy the ATS first and ask the installer to make it work afterwards.

The generator, ATS and electrical installation should be designed as one system.

What About a 63A ATS?

For a 12.5kVA single-phase generator at approximately 54A full-load current, a 63A-rated ATS can be a logical starting point because it provides a rating above the calculated full-load current.

But even here, “63A” does not automatically mean “correct”.

The device must also be suitable for:

• The system voltage
• Number of poles
• Switching duty
• Generator control system
• Mains supply
• Cable arrangement
• Protective devices
• Neutral switching requirements
• Installation environment

The actual ATS selection should therefore be confirmed against the generator’s technical documentation and the electrical installation.

Can You Use a 40A ATS With a 12.5kVA Generator?

Potentially, but only where the installation has been deliberately designed around a lower transferred load.

For example, the generator might be capable of approximately 54A at full output, while the essential-load board is protected and designed for a lower current.

However, a 40A ATS should not be installed simply because the generator is 12.5kVA. If your calculated running load is already approaching the practical limit of the 12.5kVA generator, moving up to a 14kVA generator for additional load capacity can provide useful operating headroom rather than running the smaller unit close to its maximum output.

If the connected load can exceed the ATS rating, the transfer switch can become the limiting component of the system.

The question is therefore not simply:

“What size generator do I have?”

It is:

“What current must the ATS safely transfer in this particular installation?”

That is the question an experienced installer should answer.

What ATS Size Is Best for a South African Home?

For a typical South African home using a 12.5kVA single-phase generator, the correct ATS may often fall into the 63A class, but the actual selection depends on the generator specification and the home’s electrical installation.

A common approach is to create an essential-load distribution board rather than attempting to transfer every circuit in the house. If the property’s essential circuits have a relatively modest demand, a 7kVA generator for smaller essential loads may be sufficient, particularly where high-demand equipment is excluded from the backup supply.

For homes, small offices and similar applications with carefully managed essential circuits, a 9kVA generator for residential and small-business backup can provide a useful step between smaller units and a 12.5kVA system.

For example, the backup board might supply:

Load	Backup requirement
Lighting	Usually suitable
Fridge/freezer	Usually suitable
Security system	Suitable
Wi-Fi/router	Suitable
TV	Suitable
Selected plug circuits	Depends on load
Borehole pump	Check starting current
Air conditioner	Check starting current
Electric geyser	Often excluded or separately managed

This approach can reduce the generator’s operating load and make the backup system more economical.

What About Farms, Shops and Small Businesses?

The same principle applies, but the consequences of poor sizing can be greater.

A farm may have pumps, refrigeration, electric fencing, irrigation controls and workshop equipment.

A retail shop may have refrigeration, POS terminals, lighting, security systems and air conditioning.

A small office may need computers, servers, networking equipment, security and selected HVAC systems.

Motor-driven equipment deserves particular attention because starting current can be considerably higher than normal running current.

Where several higher-demand loads may operate at the same time, a 16kVA generator for higher-demand applications may provide more practical capacity than trying to manage a 12.5kVA unit at or near full load.

A 12.5kVA generator that appears adequate from the running-load calculation may struggle when a large pump or compressor starts.

This is why ATS selection should be part of the overall generator sizing and installation process rather than treated as an afterthought.

Once the required backup load becomes substantially larger, particularly for workshops, commercial facilities, agricultural operations or industrial equipment, a 25kVA generator for commercial and industrial loads may be a more appropriate starting point than trying to stretch a 12.5kVA system beyond its practical limits.

Common ATS Sizing Mistakes

Choosing the ATS purely from the generator kVA

A 12.5kVA generator does not automatically dictate one universal ATS rating.

Avoid it: Calculate the current and confirm the generator’s actual electrical configuration.

Ignoring phase configuration

A 230V single-phase generator and a 400V three-phase generator require different considerations.

Avoid it: Confirm voltage and phase before purchasing the ATS.

Selecting an ATS that is too small

An undersized transfer switch can become a bottleneck and may be subjected to currents beyond its intended rating.

Avoid it: Select the ATS based on the actual maximum current and applicable protection requirements.

Buying the ATS before checking the generator

Some generator control systems require specific ATS interfaces, sensors or start/stop connections.

Avoid it: Confirm ATS compatibility with the generator model first.

Forgetting motor starting current

Pumps, compressors and air conditioners can produce significant startup demand.

Avoid it: Assess starting loads when sizing the complete generator system.

Attempting DIY installation

An automatic generator system involves mains electricity, generator output, changeover equipment, earthing and protection.

Avoid it: Have the installation designed and commissioned by a suitably qualified electrical professional.

South African Installation and Compliance

Generator installations should not be treated as simple plug-and-play equipment.

The electrical installation needs to comply with applicable South African requirements, including relevant wiring and electrical installation standards. The South African electrical installation standards provide the technical framework that installers need to consider when integrating generators and transfer equipment into low-voltage installations. SANS 10142-1 is the South African wiring code covering low-voltage electrical installations, and SABS identifies it as a key wiring standard.

The installation should also consider safe generator placement, ventilation, exhaust routing, earthing, isolation, cable protection and correct integration between the mains supply and generator. For a broader explanation of these requirements, see our generator installation guide.

Where the generator is being used as backup during electricity interruptions, the system should be designed so that generator power cannot inadvertently feed back into the utility network.

Expert Advice

One of the most common mistakes we see customers make is treating the ATS as an accessory rather than part of the generator installation.

The generator may be perfectly capable of supplying the required load, but the wrong transfer switch, incorrect protection or poor phase arrangement can undermine the entire system.

After helping customers assess backup-power requirements, our recommendation is straightforward: select the generator, ATS, protection and essential-load board as one coordinated system.

If you already have a 12.5kVA generator, provide the installer with the exact model and technical specification rather than simply saying that it is “12.5kVA”. That small detail can change the ATS selection.

How to Choose the Right ATS: Quick Checklist

Before purchasing an ATS for a 12.5kVA generator, confirm:

• Generator voltage
• Single-phase or three-phase configuration
• Generator rated current
• Generator standby/prime rating
• Maximum essential-load current
• Generator breaker rating
• ATS current rating
• Number of poles required
• Neutral switching requirements
• Cable sizes
• Earthing arrangement
• Generator ATS/start-stop compatibility
• Motor starting loads
• Applicable South African electrical requirements
• Installation and commissioning requirements

If any of these are unknown, it is better to establish them before ordering equipment.

Frequently Asked Questions

1. What is the correct ATS size for a 12.5kVA generator?

There is no one answer to this. A 12.5kVA single‑phase generator at 230 V can produce 54 A when it is running at full load. In that case a 63 A class ATS might fit some installations. A three‑phase 12.5kVA generator is different; the current on each phase will change,. The ATS choice must match that system.

2. Can I use a 50A ATS with a 12.5kVA generator?

Not automatically. A 12.5kVA 230 V single‑phase generator can draw 54 A at full load. A 50 A ATS would need a look at the load that will be transferred. It might not give capacity for the full power of the generator.

3. Is a 63A ATS suitable for a 12.5kVA generator?

It can be, especially if the 12.5kVA generator is set up as a 230 V single‑phase machine. The exact generator specifications and the whole installation still have to be checked before the final decision is made.

4. Does a three-phase 12.5kVA generator need a three-phase ATS?

Usually the transfer equipment must match the system that is being moved. A three‑phase generator with a three‑phase installation requires a transfer arrangement that is built for three phases.

5. Can an ATS be smaller than the generator rating?

Yes if the installation is purposely designed to transfer a smaller essential load. The ATS still has to be rated for the maximum current it will carry.

6. Does an ATS make a generator start automatically?

An ATS can initiate the automatic start sequence when it is properly connected to a generator equipped with compatible automatic-start controls. The generator and ATS must be compatible.

7. Do I need an ATS for a home generator?

Not necessarily. A manually operated changeover system can be suitable for some applications. An ATS is particularly useful where automatic backup operation is required.

8. Can I install an ATS myself?

Generator changeover installations involve mains electricity and require proper electrical protection and isolation. It is safer to have the system installed and commissioned by a suitably qualified electrical professional.

9. What happens if the ATS is too small?

The ATS can become the limiting point in the system. Depending on the installation, excessive current can result in overheating, nuisance tripping, equipment damage or a serious electrical safety risk.

10. Should the ATS be sized from the generator or the house?

Both matter. The generator establishes the available supply, while the electrical installation determines the current that the ATS must safely transfer. The final ATS selection needs to account for both.

Conclusion

The correct 12.5kVA generator ATS size cannot be selected from the generator’s kVA rating alone.

For a 12.5kVA single-phase generator operating at 230V, the full-load current is roughly 54A, which is why a 63A-class ATS may be considered in an appropriately designed installation. A three-phase 12.5kVA generator has a substantially lower current per phase, so the ATS calculation is different.

The important point is that the ATS must match the generator’s voltage, phase configuration, rated current, connected load, protection and control system.

If you are buying a 12.5kVA generator or need an ATS for an existing unit, it is worth getting the complete installation assessed before purchasing the switch. The few minutes spent checking the specifications can prevent a much more expensive installation mistake later.

If your load requirements are changing, compare the available generator capacities rather than assuming 12.5kVA will remain sufficient. Our generator sizing guide explains how to calculate running loads, starting loads and spare capacity before choosing the generator.

For assistance selecting a generator and compatible ATS for a home, farm, shop, office or commercial installation, request a generator quotation from Power Stay Solutions and provide the generator model, supply type and major equipment you need to run.